The Tories have ruled out ever signing up to an EU constitution - if the deal due to be agreed by EU leaders in June is rejected by the British people.
Mr Howard challenged the PM to a TV debate
Tory leader Michael Howard gave the pledge after Tony Blair accused him of failing to reveal his true stance.
The pair clashed during a stormy prime minister's questions on Wednesday and later exchanged letters on the issue.
Mr Howard also mocked Mr Blair's U-turn on holding a referendum and challenged the PM to a televised debate.
In a letter to Mr Blair, the Tory leader said: "If the British people were to vote yes, a Conservative government would accept the constitution.
"If the British people were to vote no, a Conservative government would veto the constitution: and we would not agree to any new treaty which establishes a constitution for the European Union."
Earlier, the Tory leader argued a televised debate would help the parties "explore all the issues".
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But the prime minister retorted that once the constitution was approved by Parliament, "we'll have the debate in the country".
MPs from both sides heckled and jeered as the two leaders slugged out their opposing positions on the EU constitution.
Mr Howard asked the prime minister when his Cabinet reversed its policy to hold a poll on the EU constitution, but Mr Blair replied: "We announced our policy on the referendum yesterday."
Veto or renegotiate?
This prompted Mr Howard to mock that despite a "huge policy U-turn" and newspaper leaks "then you tell the Cabinet there's no time left for them to make a decision".
Mr Howard asked the PM to say whether he would veto the constitution or renegotiate the treaty if the 'no' vote wins the poll.
Mr Blair said if that happened, the government would adopt a similar strategy to Ireland when it rejected the Nice Treaty for the first time, and sit down with other European countries to decide the way forward.
Later Mr Blair wrote to the Tory leader demanding to know if he would reject the constitutional treaty in its entirety or seek to amend it in discussion with the UK's European partners.
COUNTRIES HOLDING OR LIKELY TO HOLD A REFERENDUM:
Mr Howard said countries had constitutions while nation states made treaties with each other.
The government appeared ready to renegotiate the constitution in the event of a referendum 'no' vote, he said.
"In other words, if the British people did not vote the way you wanted, you would make them vote again until they did. You underestimate the British people."
Earlier, Trade Secretary Patricia Hewitt inflamed the Tories by claiming their "real argument" in campaigning for a 'no' vote in any poll was to withdraw from the EU.
STEPS TO A REFERENDUM
Constitution expected to be agreed in June
Parliamentary approval may be sought before a vote
It could also take place alongside referendums for English regional assemblies this autumn
A vote could also be held at the time of the general election predicted next spring
It is also possible a vote will take place after the next general election
A Tory spokesman said this was a "total misrepresentation" of the party's stance.
Ms Hewitt claimed the view openly shared by the right wing of the Conservative party was the same as the anti-EU UK Independence Party.
Roger Knapman, leader of the UK Independence party, said he was convinced the "no" campaign would win the battle, which would mean "we're out of the EU".
Liberal Democrat leader Charles Kennedy said the vote announcement gave an opportunity at last to "settle an issue that has bedevilled two generations of politicians".